The present invention relates to a method and a device for the electrical monitoring of an electrode lead of a bipolar high-voltage d.c. transmission system in which the electrode lead is divided into two lines at a branch point.
Systems for transmitting power by means of a high-voltage d.c. current contain two static converter stations that are connected to each other by a d.c. line. In the case of a single-pole d.c. transmission system, both stations are connected to each other by a single d.c. line, the return current being routed through ground. A d.c. pole in each station is then grounded by a good ground connection. Normally, this ground connection is located at a certain distance from the static converter station and connected to the station by an electrical line that is called an electrode lead. It can often be desirable or necessary to locate the ground connection at a great distance of up to one hundred kilometers from the station.
In the case of a double-pole d.c. transmission system, the stations are connected to each other by two d.c. current lines so that in normal operation the direct current does not need to return through the ground. For various reasons, i.e., to enable single-pole operation of the system in the event of a static converter failure, static converter stations in double-pole d.c. transmissions are provided with a ground connection that is connected to the station by an electrode lead.
An electrode lead is insulated from ground and normally is made of a multiple-wire, twisted conductor that is suspended on insulators. Although the voltage between the electrode lead and ground is normally small compared to other voltages in the system, a ground fault on the electrode lead constitutes the danger of personal injuries or damage to other system components, e.g., corrosion. It is therefore necessary to be able to discover ground faults quickly and reliably, including high-impedance ground faults and open circuits.
To locate ground faults on an electrode lead, a differential protective arrangement has conventionally been used. In a protective arrangement of this type, the current is measured at both ends of the electrode lead and a difference between the two measured currents means that a ground fault is present. However, a protective arrangement of this type has various disadvantages. It requires a communication link between the two ends of the electrode lead and therefore is expensive, especially in connection with long electrode leads. A protective arrangement of this type also does not react to a ground fault that occurs in those cases in which the electrode lead is not conducting any current, which is normally the case during operation of a double-pole transmission system without faults. Even in this case, i.e., if no direct current is flowing through the electrode lead, asymmetric currents can lead to the creation of dangerous voltages on the lead.
Ground faults on an electrode lead have also been located by injecting an alternating current signal or alternating voltage signal at a specific frequency into the lead at the static converter station. In this case suppression filters are used at both ends of the line, these filters being tuned to the frequency of the injected signal. An impedance measurement element is used to measure the impedance of the electrode lead opposite ground at the feed point at the injected frequency. A change of the impedance measured in this way is an indication of a ground fault. This method works well in cases of short electrode leads. To detect a line fault, the measurement frequency must be selected so that the length of the line is less than one quarter of the wavelength. For this reason, in the case of long electrode leads, a frequency must be selected that is so low that there is a danger of the measurement being disturbed by the mains frequency or by the lowest harmonics of the mains frequency. Furthermore, at these low frequencies, the suppression filters placed at both ends of the electrode leadxe2x80x94which must be rated for the maximum current on the electrode leadxe2x80x94become very large and expensive.
European Patent No. 0 360 109 describes a protective device for an electrode lead in which a high measurement frequency can also be used in the case of long electrode leads, thereby substantially reducing the dimensions and costs of the suppression filters as well as the danger of interference by the mains frequency or its harmonics. To prevent standing waves on the electrode lead, the suppression filter on the end of the electrode lead farthest from the feed point is provided with resistive elements that have a resistance such that the filter is matched to the characteristic impedance of the electrode lead. This prevents the measurement signal from being reflected at the far end of the electrode lead.
A method for the location of a point of fault in a cable is described in U.S. Pat. No. 5,083,086. According to this method for determining location, a repair technician executes this procedure, according to which the faulty cable is first disconnected, i.e., the cable is not in service. Next, a device used to execute the method for fault location is connected to the end of the disconnected cable. This device feeds a first electrical pulse into the cable and records the received reflections. After that, a voltage applied to the disconnected cable is increased, a second pulse is fed into the cable and the received reflections are recorded. By increasing the feed voltage, the impedance at the fault location in the cable changes so that a reflection that uniquely reproduces the fault location can be received. The recorded echo signals are compared to each other. Using this differential signal and a measured propagation time, the fault location in the cable can then be calculated.
To detect the state of an electrode lead of a bipolar high-voltage d.c. transmission system (HVDCT system), a method is described in German Patent Application No. 196 50 974.2 in which a first electrical pulse is fed to a first end of the electrode lead and an echo signal of this lead is detected. Next, a second pulse is fed into the line on the first end and its echo signal is detected. These two echo signals are then compared to each other. When there is a deviation and/or an agreement between the two echo signals, a corresponding indicator signal is generated. These process steps are continuously repeated until an error signal is generated. This indicator signal is used to stop the state measurement process. The fault location can be determined using recorded echo signals. A comparison of the faulty echo signal with stored echo signals for different operating conditions enables a more rapid determination of the error (ground fault, open circuit. . . ).
A device for performing this method has a pulse generator, an evaluation device and a coupling element. Through this coupling element, the pulse of the pulse generator is fed into the electrode lead and its echo signal is forwarded to the evaluation unit. The device is connected to one end of the electrode lead. The other end of the electrode lead is connected to ground. In order for the electrical pulse not to enter the HVDCT system, but rather only the section of the electrode lead to be monitored, the electrode lead is provided at its ends with attenuators. The evaluation device includes a comparator, a memory and a trigger device. The pulse generator-synchronized to a timer-generates rectangular, pulses with a d.c. offset. These pulses are continuously fed into the electrode lead until there is an error signal.
This method permits simple error detection in the operation of the HVDCT system without the need to use existing measurement signals. The method thus operates self-sufficiently. Since, in the fault-free case, the ground plays a part in conducting the pulse, fluctuating ground conductance affects the echo signals and thus a reliable detection of errors. Moreover, the radiation of electromagnetic energy, which is caused by the pulse in common mode, is rather high. An additional disadvantage lies in that, on both ends of the electrode lead, attenuators must be connected in series in this electrode lead. As a result, the expense for a retrofit installation in an existing HVDCT system is becoming rather high.
An object of the present invention is to provide a method for monitoring an electrode lead of a bipolar HVDCT system that no longer has the previously mentioned disadvantages and to specify an appropriate device to utilize the method.
By generating, according to the method of the present invention, a symmetric pulse signal in push-pull mode from an asymmetric pulse signal and injecting it into the two lines of the electrode lead, the ground hardly plays any further part in the transmission of these pulses, so that the method according to the present invention is nearly independent compared to a sharply fluctuating ground conductance. An additional advantage lies in that the radiation in the form of electromagnetic energy is substantially reduced compared to a common mode signal. Moreover, the push/pull mode creates a slight line attenuation so that a greater transmission range of the system accompanied by simultaneously smaller dispersion of the echo signal is allowed.
An advantage of the push/pull mode is its complete decoupling from the common mode. Interfering signals that come from the HVDCT system can propagate only in common mode since on this side of the branch point the electrode lead is combined into a single conductor and thus an electromagnetic field can exist only between this conductor and ground. Interfering signals coming from the HVDCT system propagate to the electrode lead nearly at the speed of light, are divided at the branch point with nearly identical amplitude and phase and then travel on the two waveguides, namely conductor-ground and conductor-ground, to the end of the electrode lead most distant from the system. However, between the feed connections mounted at equal distances from the branch point, these interfering signals cannot generate any voltage, leading to an ideal, frequency-independent decoupling of the method for monitoring the electrode lead from the HVDCT system. On the other hand, because of the reciprocity of the electrode lead, no signals that are fed to the feed connections in push/pull mode reach the HVDCT system, resulting in the method being independent of whatever circuit states may exist in the HVDCT system. In order to be able to inject a signal in push/pull mode into the electrode lead composed of two lines, the short circuit for this mode is made inactive in the branch point. This could, for example, be brought about by connecting a high-inductance coil in series in the electrode lead between each of the feed connections and the branch point. Since, in single-pole mode, currents in the kA order of magnitude flow through the electrode lead, the two coils needed for this would also be designed for these currents.
An example embodiment of the method according to the present invention provides for implementing the injection of pulse signals in push/pull mode without such components as the coils mentioned. This is possible if the feed points are located at a predetermined distance from the branch point, this distance being sized so that it corresponds to approximately one quarter of the conduction wavelength at the center frequency of the generated unbalanced-to-ground pulse. At this frequency, the short circuit is transformed in the branch point into an open circuit at the feed connections, and at adjacent frequencies this short circuit is transformed into a high-impedance reactance, which, at the feed connections is to be considered as connected in parallel to the characteristic impedance of the line.
Another advantage of the method according to the present invention lies in that this monitoring procedure can independently adapt to different operating conditions. This is achieved by generating an echo difference curve as a function of a recorded actual echo curve and a stored, constructed dynamic target echo curve. By using a dynamic target echo curve that can vary over time, influences of the seasons on the electrode leads are taken into consideration so that an error case can be uniquely determined for each.
If an error signal is generated, then the monitoring process can be switched off. To do this the generation of pulses is interrupted or switched off.
In an advantageous embodiment of the method according to the present invention, a given static target echo curve is also generated and flanked by a tolerance band, which is determined by a limit curve running above and below this static target echo curve. A constructed dynamic target echo curve is then checked in relation to this static target echo curve as to whether at least one amplitude of this dynamic target echo curve is outside of the tolerance band of the static target echo curve. If this is the case at least once within a given time frame, an error signal is generated and the monitoring process is turned off. By using a given static target curve, defects can be detected in the device for monitoring the electrode lead that, if they occur gradually, would otherwise fall under an operating state that changes over time.
An additional advantageous embodiment of the method according to the present invention provides for the construction of the dynamic target echo curve from a mean value of at least two consecutive actual echo curves. In other words, a mean value is continually formed from a given number of consecutive actual echo curves and is stored as a dynamic target echo curve. As a result, with each new actual echo curve, a new mean value is stored as a dynamic target echo curve. However, this only happens if no indicator signal was generated during the evaluation of an echo difference curve.
A symmetric pulse signal in push/pull mode is generated from an asymmetric pulse signal that is generated by the pulse generator due to the availability of a feed device that is linked on the output side with each feed connection of the two lines of the electrode lead in the device provided for the utilization of the new monitoring method along with a pulse-echo monitoring device that has a pulse generator and a receiving unit. The pulse-echo monitoring device is linked to the inputs of the feed circuit. This feed circuit has on its input side a device for pulse conversion and on its output side two coupling capacitors that connect each of the outputs of the device for pulse conversion to a feed connection.
Due to the configuration of the feed device, first, a symmetric pulse signal in push/pull mode is generated from an unbalanced-to-ground pulse signal of the pulse generator, thereby introducing the previously mentioned advantages of push/pull mode over common mode and, secondly, interference that comes from the HVDCT system is transmitted only in a very sharply attenuated state to the receiving unit.
In an advantageous embodiment of the feed circuit, an isolation transformer with low-voltage and high-voltage windings, two coils and two diverters are provided as equipment for pulse conversion, one coil and one diverter being connected in parallel with each high-voltage winding. The connection point of the two high-voltage windings is connected to ground potential. The two coupling capacitors in conjunction with the two coils form two high-pass filters, which are each tuned to the center frequency of the generated pulse. The diverters protect the isolation transformer against excess voltages in the event of transient interference (lightning strike, switching surge).
According to another advantageous embodiment of the device according to the present invention, the pulse generator has two voltage sources, two capacitors, two switches, two resistors and one actuating device for the switches, each capacitor being electrically connected to a voltage source via a resistor so as to conduct. One connection point of these two capacitors and one connection point of the two voltage sources are each connected to ground potential. The capacitors can each be linked through a switch to the output of the pulse generator, the actuation device being connected to a control output of the pulse generator. Using a pulse generator of this type, a narrow-band, rectangular pulse with no d.c. offset and having a high spectral component at its center frequency is produced.
In principle, still other pulse shapes having the spectral characteristics already mentioned can be used for the new monitoring method. For example, a saw-tooth pulse running symmetrically with respect to the time axis can also be used. However, the generation of a pulse of this type is more complicated.
According to an additional beneficial embodiment of the new device, the receiving unit has a device for the real-time recording of echo signals, a processing unit, a main memory and an input and output interface, the control input of this feed unit being connected to the control input of the device for real-time recording of echo signals. The processing unit is connected to the main memory, the device for real-time recording and the interfaces. One signal input of the device for real-time recording is connected to the input of the receiving unit, the input and output side of a master system controller being connected to the output and input interfaces.
By connecting the control output of the pulse generator to the control input of the device for real-time recording, this device is triggered by the output of pulses by the pulse generator. In this way, the echo signals can be recorded for a given time, i.e., this part of the receiving unit is operated online. Further processing of these recorded echo signals occurs offline, this additional processing being handled centrally in the processing unit.
In the new device, the feed connections of the lines of the electrode lead are each located by design at a distance from a branch point of the electrode lead, this distance being in particular equal to one quarter of the free-space wavelength of the center frequency of the pulse. Through the selection of the distance of these feed connections from the branch point, circuit elements must not be connected in series in the electrode lead.
For the center frequency of the feed pulse, the short circuit in the branch point of the electrode lead is transformed over the xc3xa/4 line into an open circuit at the feed location. The xc3xa/4 length line together with the entire HVDCT system is thus electrically not present at this frequency. The supplied pulse at this frequency sees only the characteristic impedance of the two lines of the electrode lead that run to the ground electrode and to the branch point. At other frequencies, the short circuit in the branch point is transformed by way of the line, which is then no longer xcex/4 long, into a reactance that can be considered connected in parallel to the characteristic impedance of the line at the feed location.
By utilizing system-side factors and by activating push/pull mode, no additional circuitry measures are needed to decouple the measurement arrangement from the station. In this way, expensive attenuation elements are unnecessary.
Additional beneficial embodiments of the device for monitoring an electrode lead of a bipolar HVDCT system can be seen in dependent claims 13 through 19.